In the art of electrical transmission, conductors are repeatedly coupled so that electrical current flows from one conductor to another through some type of electrical interface. An electrical wire, for instance, often is connected to another electrical wire, to a conductor on a flat circuit such as a flat flexible circuit, to a circuit trace on a printed circuit board or in all kinds of combinations of such conductors. Often, two respective conductors are interconnected or electrically coupled by a soldering material. The solder is heated to its melting point and, when solidified, the solder mechanically and electrically or conductively joins the two conductors.
For example, a flat flexible circuit typically includes a plurality of flexible conductors which often are generally parallel to each other similar to the electrical wires of a hard-wired electrical cable. The flexible conductors are in a plane and are adhered to a flexible insulating substrate, such as an elongated substrate. Most often, the conductors are sandwiched between a pair of flexible insulating layers or films. The films and conductors are held in their flat configuration by an appropriate adhesive. When it is required to connect the conductors of a flat flexible circuit to the conductors of another circuit or other conducting member, the insulating substrate (i.e., the insulating layer) on one side of the flat circuit is removed to expose the embedded conductors. In some instances, one of the insulating layers on one side of the circuit is left shorter than the insulating layer on the opposite side of the circuit to expose distal ends of the conductors for connection to whatever connecting device(s) the circuit is to be electrically coupled.
Problems have been encountered in coupling such conductors as the flexible conductors of a flat circuit, particularly when the conductors are soldered and which requires the application of heat. In essence, the solder must be melted without melting the thin insulating films. Additionally, the parallel flexible conductors often are of varying widths in the same flat circuit and the resulting varying densities cause heat distribution problems. For instance, the heat required for soldering may be so intense in one area as to melt the insulating substrate or film and yet be insufficient in another area that a conductor or conductors are not adequately electrically coupled.
Various prior art methods have been used to electrically bond the conductors of flat circuits by soldering techniques. Such methods have included diode laser soldering and pulsed hot bar soldering. Laser soldering requires that several spot solder joints be made and scanned across the joint and this is a relatively time consuming process. Laser soldering requires expensive automation to focus the energy. Laser soldering also requires that the insulating substrate or film of the flat circuit to be transparent, and this is a problem with flame retardant material films that have opaque fillers as well as with opaque adhesives used in fabricating flat flexible circuits. Finally, laser soldering may require expensive stencils for soldering flat circuits.
Pulsed hot bar soldering processes also have had limited success in bonding the conductors of flat circuits. This process is not capable of heating relatively large conductors without melting the insulating substrates or films. Like laser soldering, the hot bar makes it difficult to focus energy to selective areas. The hot bar method relies on conduction and convection rather than a more desirable induction technique. Finally, heating heads for hot bar soldering apparatus are quite expensive.
The invention is directed to solving the above myriad of problems in an induction soldering process which uses unique magnetic concentrators. As is known, inductive coils create magnetic lines which pass through metal and cause eddy currents which, in turn, create friction and heat. The invention utilizes very simple and inexpensive magnetic concentrator blocks to selectively concentrate this heat wherever desired to melt the solder very quickly and evenly and bond the conductors without melting or damaging the insulating substrates or films.